The invention relates to a multi-component electrically heatable backing means for mirrors, comprising thin metal sheet means and at least one plate-shaped electrical heating element having two main faces being connected with said metal sheet means in an electrical current- and heat-conducting manner.
Such heatable backing means are, for instance, known from German Pat. No. 29 19 968. They serve in particular for heating the external mirrors of motor vehicles, but can also be used for other mirrors such as, for instance, bathroom mirrors. The metal sheet means serve for distributing over the entire surface of the mirror heat which is generated in a small surface area by the heating elements, thereby preserving the mirror at least substantially free from fogging or ice formation.
It is particularly desirable in the case of the external mirrors of motor vehicles that the center of such mirror is heated first, thereby ensuring that at least a part of the mirror surface of an ice-covered mirror becomes usable already after a short heating period. The desired heat-distribution can be attained by a corresponding dimensioning of the thickness of the backing material as well as the width of the heatable backing means.
Heatable backing means of the above-described kind are usually produced from thin metal sheet, for instance copper sheet or aluminum foil. The sheets or foils are glued on to the back of a mirror. Heating elements mostly used consist of small plate-shaped bodies of ceramic masses containing a resistor whose resistance increases with increasing temperature over the temperature range in question (so-called PTC elements). The metal sheets or foils serve at the same time as electrical current feed-in to the heating elements, wherefor their arrangement must be such that the metal sheets or foils should cover the two main faces of a plate-shaped heating element, i.e., the two flat faces, and be in current- and heat-conducting connection therewith.
In the case of German Pat. No. 29 19 968, the metal sheets are of such shape and so arranged that they overlap with formation of stripes, the heating elements being arranged between the overlapping zones of the sheets.
The known heatable backing means for mirrors suffer from the drawback that they must be specifically cut to fit the size of the mirror. This leads to disadvantages during manufacture, when the backing means are to be applied simultaneously with the manufacture of the mirror. But there ensue even more significant drawbacks for the manufacturer and the dealer, and last not least for the user, when the heatable backings are offered in commerce as retro-fitting parts, which are to be glued by the user himself on to the back of a mirror. In this case, the maintenance of stores of a great variety is necessary in order to cover mirrors of any size likely to occur. Moreover, the user may have to acquire several sizes, if he wants to equip different mirrors with heatable backings.